Generally, IC devices utilize various frequency sources in generating clock signals for use by different logic and/or analog circuits in the IC devices. High-Q filters can be used to create oscillators by incorporating them in a positive feedback loop with amplifiers providing adequate gain. Such oscillators are essential as a signal source for communication systems as well as analog electronics. They can also be used as a clock source for digital electronics. High-Q filters are also used in communication systems to select specific bands and channels, eliminate interferers, and suppress spurious transmissions, among many other uses. The higher the quality factor Q of the filter, the better selectivity it provides for different channels and bands, as well as lower phase noise and jitter for the oscillators constructed by such filters. However, the scaling to GHz-frequency bands and even 10's of GHz, renders the design of high-Q filters very challenging. Further, complete system integration on chip is becoming the only way to meet the ever increasing demand for the reduction of size, weight and power consumption of electronic systems.
The ability to construct inductors from CMOS metallization layers allows for creation of on-chip LC tank circuits. These are monolithically integrated electrical resonators and can easily achieve 10's of GHz frequencies. However, they suffer from low-quality factor (Q<50) and high electrical losses. In addition, on-chip inductors usually occupy a large die area that cannot be populated by other devices, hence increasing the overall system cost. Quartz crystals have dominated the high-precision oscillators market for half a century owing to their high-quality factors (Q˜105), low insertion loss and low temperature coefficient. However, Quartz crystals are limited to a few 100 MHz and are difficult to scale to GHz frequencies. Their integration in CMOS ICs has proven to be quite challenging due to the incompatible fabrication processes.
Monolithic integration of mechanical resonators for filter applications into CMOS ICs has been the focus of multiple research efforts over the past 3 decades. Filters relying on mechanical resonators, show superior performance over the electrical LC tank circuits. Micro-electro-mechanical systems (MEMS) resonators with quality factors often exceeding 104, small size (1000× smaller than on-chip LC tanks), and the capacity for integration with CMOS circuits make them a potential solution for current timing and RF challenging demands. MEMS devices and resonators usually incorporate moving surfaces. Some of these surfaces must be free boundaries, and often the entire MEMS device has to be suspended. Hence MEMS devices usually incorporate sacrificial layers that are used to support the device during the fabrication process and then are etched away in a release process to create a freely suspended device. Multiple efforts to integrate MEMS resonators in CMOS processes are available, including MEMS first, MEMS last, MEMS above IC and CMOS-MEMS. With the MEMS first technique, the MEMS device is fabricated before the CMOS circuit and protected by passivation layers during the CMOS circuits processing. When CMOS processing is done, the MEMS device is exposed and released by etching the sacrificial layers. The MEMS last technique relies on fabricating the CMOS circuits first. When the CMOS circuit is complete, it is passivated and protected when the MEMS device is fabricated. The MEMS device is then released to create the required freely suspended structures. In both these techniques, the MEMS device ends up consuming valuable CMOS area.
MEMS above ICs technique relies on growing the MEMS device on top of the completed CMOS die, followed by the necessary release steps. In all the above mentioned techniques, the sequential processing the MEMS and CMOS always results in a limited thermal budget for one or the other. Also, the CMOS or MEMS yield may be significantly compromised by virtue of the extra processing. CMOS-MEMS devices are used to refer to the devices formed by patterning and etching the back-end-of-line (BEOL) layers of the completed CMOS die. The CMOS-MEMS BEOL fabrication technique has been successfully applied to MEMS resonators in the low MHz range. Scaling to GHz-frequencies is a challenge, since it requires smaller dimensions, whereas the CMOS BEOL process usually have large critical dimensions (CD) compared to the front-end-of-line (FEOL) process. CMOS-MEMS BEOL devices usually incorporate large air gaps as defined by the BEOL CD, which dictates higher operating voltage (10's of Volts) for the MEMS device, and complicates the interfacing with CMOS circuits.
Film Bulk Acoustic wave Resonators (FBAR) are another variant of MEMS resonators which is widely adopted as filters in the RF industry. FBARs offer high-Q and low insertion loss, but their resonance frequency is determined by the layer thickness as they are thickness-mode devices. This limits their application to a single frequency per wafer in the case of integration with CMOS.
Some IC devices may utilize RBTs as an onchip frequency source providing higher frequency and lower phase noise improvements over traditional solutions such as inductor-capacitor (LC) tanks, quartz crystals, or FBARs. CMOS RBTs are unreleased MEMS resonators implemented as an integral part of the CMOS FEOL and BEOL process, without any extra release or passivation steps. They are fabricated just like any regular FET in the CMOS process. With the lack of a release step and extra post-processing, CMOS RBTs do not compromise the yield of the CMOS process or the RBT itself. Also, as unreleased devices with no air gaps, they are inherently encapsulated in the CMOS die and do not require any special packaging or hermetic sealing. CMOS RBTs incorporate a mechanical resonance cavity located in the FEOL layers of the CMOS process. The RBT resonance cavity is defined from the top by 1D, 2D or 3D phononic crystal (PnC) formed from the metal and dielectric BEOL layers of the CMOS processes. Total internal reflection from the CMOS bulk wafer is used to achieve energy confinement from the bottom, which together with the PnC define the cavity vertical dimensions. Patterning of FEOL layers is used to construct in-plane reflectors achieving horizontal energy confinement and define the horizontal cavity dimensions. CMOS RBTs make use of a regular FET from the CMOS technology for active FET sensing. The mechanical stresses in the acoustic resonance cavity modulate the mobility of the FET channel resulting in a small signal current in the external circuit when the FET is biased properly.
However, the current RBTs use capacitive driving and sensing methods that generate weak (e.g. 1 micro-siemens (μS)) frequency signals requiring amplification by a trans-impedance amplifier to obtain an oscillating/clock signal. CMOS RBTs are electrostatically driven by the help of MOS Capacitors (or regular FETs used as capacitors) available in CMOS FEOL. Modulation of the charge on the MOS capacitor (through the gate voltage) causes a modulation of the electrostatic voltage induced by these charge, and inducing mechanical stress in the structure. CMOS RBTs benefit from the small CDs available for the FEOL of CMOS and are inherently scalable to GHz-frequencies. CMOS RBTs are also small in size spanning only few micrometers, hence they do not consume costly die area. With the CMOS RBTs available directly in the FEOL of the CMOS die, interconnects parasitic to CMOS circuits are minimal compared to any other integration scheme. However, methods to reduce/prevent propagation of the acoustic energy into different parts (e.g. substrate) of an IC device may limit availability of different frequency signals that may be produced by the RBTs.
Therefore, a need exists for a methodology enabling utilization of RBTs to generate and sense higher frequency signals with high quality factors (Qs) in an IC device and the resulting device.